In the semiconductor industry it is common to obtain overlay metrology measurements of integrated circuit (IC) fabrication substrates during fabrication to insure that they are within the necessary alignments tolerances.
Such overlay metrology marks generally comprise features formed in two layers, the features configured to enable measurement of spatial displacement between features of the layers (i.e., the overlay or displacement between layers).
Many conventionally methods of conducting overlay measurements are known. And each serve well enough for their own purposes. However, certain improvements can be made.
Conventional overlay metrology marks generally comprise two features, one in each layer, which are positioned side by side or one inside the other. Examples of such approaches are outlined at many places in the art. The following references illustrate various examples of prior art approaches. One example is “In-chip overlay measurement by existing bright-field imaging optical tools” by Yi-Sha Ku, Chi-Hong Tung, and Nigel P. Smith, Proc. SPIE 5752, 438 (2005). Another example, is “Overlay measurement tool up to 70-nm design rule” by Tatsuo Fukui, Hiroshi Aoki, Takeshi Endo, and Tomoaki Yamada. Another is disclosed in “In-chip overlay metrology in 90 nm production” by Bermd Schulz, Rolf Seltmann, Joerg Paufler, Philippe Leray, Aviv Frommer, Pavel Izikson, Elyakim Kassel & Mike Adel, ISSM 2005.
These examples are merely illustrative of aspects of the prior art and are not in any way limiting to the invention as disclosed herein.
One common prior art implementation is the so-called “box-in-box” metrology mark which employs marks in two layers subject to imaging techniques and image processing to obtain direct measurement of offset between the features of each layer and a comparison of those measurements to an intended offset to determine if the layers in question are within an alignment specification. Such targets are quite large and are not easily adaptable to the constant need in the art of implementing smaller and smaller targets.
Another overlay metrology imaging target 100 is a so-called side-by-side grating AIM type target. FIG. 1 provides a simplified illustration of one embodiment of such a target. A first set of gratings (schematically depicted by alternating dot-dashed lines) 101-104. Are formed in a first layer L1 of the target 100. A second set of gratings 111-114 is formed in a second layer L2 (schematically depicted bylines). Each grating in the first layer L1 is accompanied by a complementary target in the second layer L2 that is position to the side of the first target. Such pairs are arranged side by side so that each grating in L1 is parallel to an associated grating L2. Pairs 101/111, 102/112, 103/113, 104/114 each illustrate an example of such side-by-side pairs. These gratings are imaged and measurements of overlay are obtained (e.g., by comparing the displacement between adjacent side by side gratings such as 101 and 111). One problem inherent in the side-by-side imaging target 100 is that such targets are large, commonly being in the range of about 20 μm×20 μm to about 40 μm×40 μm in size. Moreover, because the gratings are adjacent to one another they very susceptible to cross talk from adjacent gratings. Thus, in order to prevent such cross-talk from degrading the resultant signal “exclusion zones” at the interfaces between adjacent gratings are used to prevent excessive illumination of adjacent grating structures. Illustratively, FIG. 1 depicts some examples of such exclusions zones. Exclusion zones 121, 122, 123, & 124 are depicted to illustrate some examples. The targets, of course, include still more exclusion zones. The inspector cannot illuminate the exclusion zone due to the risk of contaminating the signal. As a consequence, such targets not only suffer from large size and risk of contamination in the signal, but also suffer from the fact that large portions of the target (the exclusion zones) cannot be used to obtain metrology measurements. As briefly discussed, such targets have finite limits on how small they can be made (at least due to the need for the “exclusion zones”) and have less than desirable suitability for being made smaller. Another shortcoming inherent in such side-by-side gratings like those in FIG. 1, is the problem of irregular contrast between target gratings. Because the targets are formed in different layers, each portion of the target and each layer of the target is subject to varying optical properties which makes contrast equalization between the various layers and gratings a daunting and difficult problem not easily overcome in the present art.
In addition to imaging metrology targets (such as those discussed above) so-called scatterometry overlay (SCOL) targets are employed to obtain overlay metrology measurements. Commonly, SCOL targets are employed as a targeting group with each target comprising a pair of two parallel gratings formed one over the other in different layers with a predetermined and deliberate offset between the gratings of the different layers. Additionally, the targeting groups include targets with gratings parallel to an “x” axis and other targets with gratings parallel to a “y” axis. Each target features a pair of gratings offset with respect to each other to enable accurate overlay metrology measurements. The intensity of a light scattering signal produced by illuminated targets can be used to determine overlay errors. Common examples of such intensity measurements are measurements of intensity at selected wavelengths or intensity measurements made at different scattering angles. In common implementations, such scatterometry targets are generally employed in target groups of eight or more adjacent targets. Thus, these SCOL targeting groups are quite large and also have many exclusion zones (e.g., in an eight target group ten exclusion zones can be present). Thus, such SCOL targets also suffer from finite size limitations and large exclusion zones that hinder there usage in ever smaller dimensions.
Thus, although existing targets and target measurement approaches are suitable for many implementations, the inventors suggest that many improvements can be made. The invention described herein discloses method and target apparatus for enabling improved metrology measurements.